**Core Concept**
The patient's symptoms suggest an inflammatory condition affecting the orbit, specifically involving the lacrimal gland and surrounding structures. This condition is likely a manifestation of a systemic disease process, given the patient's history of repeated pain over the medial canthus and chronic use of nasal decongestants.

**Why the Correct Answer is Right**
The patient's presentation is consistent with orbital cellulitis or a more severe inflammatory condition, such as orbital pseudotumor or a vasculitic process. However, the most likely diagnosis in this case is a complication of a chronic condition, specifically **Thyroid Ophthalmopathy**. The patient's symptoms, including diplopia, proptosis, and chemosis, are classic for this condition. The congested optic disc suggests increased intraocular pressure, which is a known complication of Thyroid Ophthalmopathy. The history of repeated pain over the medial canthus and chronic use of nasal decongestants may be related to a pre-existing condition, possibly a thyroid disorder.

**Why Each Wrong Option is Incorrect**
**Option A:** While orbital cellulitis is a possible diagnosis, the patient's history and symptoms suggest a more chronic condition. Orbital cellulitis typically presents with acute onset of symptoms and is often associated with a facial fracture or sinusitis.
**Option B:** Orbital pseudotumor is a condition that can mimic Thyroid Ophthalmopathy, but it typically presents with more severe symptoms, including severe pain and vision loss. The patient's history of repeated pain over the medial canthus and chronic use of nasal decongestants makes Thyroid Ophthalmopathy a more likely diagnosis.
**Option C:** A vasculitic process, such as giant cell arteritis, can cause orbital symptoms, but it is typically seen in older adults and is often associated with headaches and jaw claudication.
**Option D:** A lacrimal gland tumor or cyst is unlikely given the patient's symptoms and history.

**Clinical Pearl / High-Yield Fact**
Thyroid Ophthalmopathy can cause a range of orbital symptoms, including proptosis, chemosis, and diplopia. It is often associated with a pre-existing thyroid disorder, such as Graves' disease.

**Correct Answer:** D.